A hub unit is known, which includes a direct-current motor and is mounted on a wheel of an electrically assisted bicycle (see, for example, JP-A-2005-335535).
A hub unit disclosed in JP-A-2005-335535 essentially includes a motor including a stator and a rotor, and a hub accommodating the motor therein and rotatively driven by rotation of the rotor. A wheel on which the hub unit is mounted is rotated by the rotative driving of the hub.
The motor includes a motor housing (motor case) which retains the stator. The motor housing includes a pair of end plates disposed in opposed relation on opposite sides of a clad metal plate of the stator. The end plates are clamped against each other by bolts with the clad metal plate being held therebetween, whereby the stator is retained in the motor housing with its outer peripheral surface being open to an interior of the hub between the pair of end plates. In this case, even if coils of the stator generate heat, the overheat of the stator can be prevented by releasing the heat out of the motor housing between the pair of end plates.
The end plates respectively include bearings, which rotatably support opposite end portions of a rotation shaft of the rotor.
In the motor of the hub unit disclosed in JP-A-2005-335535, the clad metal plate of the stator is held between the pair of end plates of the motor housing. Therefore, the motor is liable to suffer from dimensional variations occurring in an opposing direction of the pair of end plates depending on the state of the clad metal plate clamped between the pair of end plates and an error in the thickness of the clad metal plate. The variations result in positional offset of the bearings of the respective end plates (in other words, the variations impair the parallelism between the bearings). If the bearings rotatably support the rotation shaft of the rotor with the positional offset, the bearings are liable to be strained.